Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

The old struct config_param remains only for top-level string options.

Require the version 2 which has a more sane API.

For free().

Replaces the method Close().

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.